Fabricating optical waveguide gratings

ABSTRACT

A method of fabricating an optical waveguide grating having a plurality of grating lines of refractive index variation comprises the steps of: (i) repeatedly exposing a spatially periodic writing, light pattern onto a photosensitive optical waveguide, and (ii) moving the writing light pattern and/or the waveguide between successive exposures of the writing light pattern, so that each of at least a majority of the grating lines is generated by at least two exposures to different respective regions of the writing light pattern.

[0001] Dispersion compensation is an attractive technique allowing theupgrade of the existing installed standard fibre network to operation at1.5 μm where it exhibits a dispersion of ˜(about) 17 ps/nm.km whichwould otherwise prohibit high capacity (eg. 10 Gbit/s) datatransmission.

[0002] Chirped fibre gratings are currently the most attractivetechnique for fibre dispersion compensation [1]. This is because theyare generally low loss, compact, polarisation insensitive devices whichdo not tend to suffer from optical non-linearity which is the case withthe main competing technology, dispersion compensating fibre.

[0003] For present practical applications chirped gratings must exhibitboth high dispersion, ˜1700 ps/nm, sufficient to compensate thedispersion of around 100 km of standard fibre at a wavelength of 1.55μm, and a bandwidth of around 5 nm. This implies a need for a chirpedgrating of length 1 m.

[0004] Fibre gratings are generally created by exposing the core of anoptical fibre to a periodic UV intensity pattern [2]. This is typicallyestablished using either an interferometer or a phase mask [3]. To date,phase masks are the preferred approach owing to the stability of theinterference pattern that they produce. The length of the grating can beincreased by placing the fibre behind the phase mask and scanning the UVbeam along it. Techniques for post chirping a linear grating afterfabrication include applying either a strain [1] or temperature gradient[4] to it. However these techniques are limited due to the length of theinitial grating (˜10 cm with available phase masks) and the length overwhich a linear temperature or strain gradient can be applied.Alternatively more complex step chirped phase masks can be employed [5].However, all of these techniques are currently limited to a gratinglength of about 10 cm.

[0005] In addition to chirping the grating, it is also sometimesdesirable to be able to apodise (window) the gratings to reduce multiplereflections within them and to improve the linearity of the time delaycharacteristics. A powerful technique has been developed which allowschirped and apodised gratings to be written directly in a fibre,referred to as “the moving fibre/phase mask scanning beam technique”[6]. This technique is based on inducing phase shifts between the phasemask and the fibre as the phase mask and fibre are scanned with the UVbeam. Apodisation is achieved by dithering the relative phase betweenthe two at the edges of the grating. Like all the previous techniquesthe one draw back with this technique is that it is again limited togratings the length of available phase masks, ˜10 cm at present.

[0006] This problem has been overcome in one approach by Kashyap et alusing several 10 cm step-chirped phase masks [5]. These are scanned inseries to obtain a longer grating. The phase “glitch” or discontinuitybetween the sections is subsequently UV “trimmed” to minimise itsimpact. However this is a time consuming and costly process. In additionthe effect of the UV trimming will vary with grating ageing.

[0007] A technique for potentially writing longer gratings has beenreported by Stubbe et al [7]. In this case a fibre is mounted on anair-bearing stage and continuously moved behind a stationary gratingwriting interferometer. The position of the fibre is continuouslymonitored with a linear interferometer. The UV laser is pulsed to writegroups of grating lines with period defined by the writinginterferometer. A long grating can be written by writing several groupsof grating lines in a linearly adjacent series, with controlled phasebetween the sections. The phase shift between each group of gratinglines is controlled via the linear interferometer and a computer whichsets the time the laser pulses. A short pulse, ˜10 ns, is required suchthat the position of the writing lines is effectively stationary andaccurately controlled with respect to fibre motion. Having said this,however, jitter in the pulse timing and in the linear interferometerposition will give detrimental random phase errors in the grating.Chirped gratings can potentially be fabricated by continuouslyintroducing phase shifts between adjacent groups along the grating.Obviously the maximum translation speed is limited by the number ofgrating lines written with one laser pulse and the maximum repetitionrate of the pulsed laser. It is also proposed in this paper thatapodisation is achieved by multiple writing scans of the grating.

[0008] This invention provides a method of fabricating an opticalwaveguide grating having a plurality of grating lines of refractiveindex variation, the method comprising the steps of:

[0009] (i) repeatedly exposing a spatially periodic writing lightpattern onto a photosensitive optical waveguide; and

[0010] (ii) moving the writing light pattern and/or the waveguidebetween successive exposures or groups of exposures of the writing lightpattern, characterised in that the successive exposures or groups ofexposures overlap so that each of at least a majority of the gratinglines is generated by at least two exposures to different respectiveregions of the writing light pattern.

[0011] Embodiments of the invention provide a number of advantages overprevious techniques:

[0012] 1. The realisation that the laser does not have to be pulsed butjust has to be on for a particular duty cycle—preferably less than 50%of the period. This allows an externally modulated CW (continuous wave)laser to be used.

[0013] 2. With this technique the grating lines are re-written byseveral successive exposures of the writing light beam at every gratingperiod (or integral number of grating periods). Thus the footprintdefined by the writing light beam is significantly overlapped with theprevious lines. Significant averaging of the writing process is achievedthus improving the effective accuracy and resolution of the system,compared to that of [7] where a group of lines is written in a singleexposure, and the fibre is then advanced to a fresh portion where afurther group of lines is written in a single exposure.

[0014] 3. Effectively controlling the grating writing process on aline-by-line basis allows accurate apodisation to be achieved. This maybe performed in embodiments of the invention by dithering the gratingwriting interferometer position in the fibre to wash out or attenuatethe grating strength whilst keeping the average index change constant.

[0015] 4. The technique offers the further advantage that the CW lasermay be extremely stable, whereas pulsed lasers (e.g. those used in [7])may suffer from pulse-to-pulse instability which is not averaged. Inaddition the high peak powers of the pulsed laser may cause non-lineargrating writing effects.

[0016] 5 Arbitrary phase profiles and in particular a linear chirp canbe built up by inducing phase shifts electronically along the grating asit grows. In a similar manner to the “Moving fibre/phase mask” technique[6] the maximum wavelength is inversely proportional to the beamdiameter. This can be further improved in particular embodiments of theinvention by incorporating a short, linearly chirped phase mask. Thus asthe fibre is scanned the UV beam may be also slowly scanned across thephase mask, an additional small phase shift is induced, whilst mostsignificantly we have access to writing lines of a different periodallowing larger chirps to be built up.

[0017] This invention also provides apparatus for fabricating an opticalfibre grating having a plurality of grating lines of refractive indexvariation, the apparatus comprising:

[0018] a writing light beam source for repeatedly exposing a spatiallyperiodic writing light pattern onto a photosensitive optical waveguide;and

[0019] means for moving the writing light pattern and/or the waveguidebetween successive exposures or groups of exposures of the writing lightpattern, characterised in that

[0020] the successive exposures or groups of exposures overlap so thateach of at least a majority of the grating lines is generated by atleast two exposures to different respective regions of the writing lightpattern.

[0021] The various sub-features defined here are equally applicable toeach aspect of the present invention.

[0022] The invention will now be described by way of example withreference to the accompanying drawings, throughout which like parts arereferred to by like references, and in which:

[0023]FIG. 1 is a schematic diacram of a fibre grating fabricationapparatus;

[0024]FIGS. 2a to 2 c are schematic diagrams showing a gratingfabrication process by repeated exposures;

[0025]FIGS. 3a and 3 b are schematic timing diagrams showing themodulation of a UV beam; and

[0026]FIGS. 4a and 4 b are schematic graphs characterising a 20 cmgrating produced by the apparatus of FIG. 1.

[0027]FIG. 1 is a schematic diagram of a fibre grating fabricationapparatus. An optical fibre (e.g. a single mode photorefractive fibre)10 is mounted on a crossed roller bearing translation stage 20 (such asa Newport PMLW160001) which allows for a continuous scan over 40 cm. Thefibre 10 is positioned behind a short (˜5 mm) phase mask 30 (e.g a maskavailable from either QPS or Lasiris).

[0028] The fibre is continuously and steadily linearly translated orscanned in a substantially longitudinal fibre direction during thegrating exposure process.

[0029] Ultraviolet (UV) light at a wavelength of 244 nm from a CoherentFRED laser 40 is directed to the fibre/phase mask via an acoustic-opticmodulator 50 (e.g. a Gooch & Housego, M110-4(BR)) operating on the firstorder.

[0030] The relative position of the fibre to the interference pattern ofthe phase mask is continuously monitored with a Zygo, ZMI1000differential interferometer 55. The interferometer continuously outputsa 32-bit number (a position value) which gives the relative positionwith a ˜1.24 nm resolution. This output position value is compared by acontroller 70 with switching position data output from a fast computer60 (e.g. an HP Vectra series 4 5/166 with National InstrumentsAT-DIO-32F) in order that the controller can determine whether the UVbeam should be on or off at that position. Whether the UV beam is infact on or off at any time is dependent on the state of a modulationcontrol signal generated by the controller 70 and used to control theacousto-optic modulator 50.

[0031] So, as each position value is output by the interferometer, thecontroller 70 compares that position value with the switching positiondata currently output by the computer 60. If, for illustration, theinterferometer is arranged so that the position values numericallyincrease as the fibre scan proceeds, then the controller 70 detects whenthe position value becomes greater than or equal to the currentswitching position data received from the computer 60. When thatcondition is satisfied, the controller 70 toggles the state of themodulation control signal, i.e. from “off” to “on” or vice-versa. At thesame time, the controller 70 sends a signal back to the computer 60requesting the next switching position data corresponding to the nextswitching position.

[0032] If the fibre was scanned with the UV beam continuously directedonto the fibre, no grating would be written since the grating lineswould be washed out by the movement.

[0033] However if the UV beam is strobed or modulated (under control ofthe switching position data generated by the computer 60) with a timeperiod matching or close to:$\frac{{phase}\quad {mask}\quad {projected}\quad {fringe}\quad {pitch}}{{fibre}\quad {translation}\quad {speed}}$

[0034] then a long grating would grow.

[0035] This expression is based on a time period of a temporally regularmodulation of the UV beam, and so assumes that the fibre is translatedat a constant velocity by the translation stage. However, moregenerally, the switching on and off of the UV beam is in fact related tothe longitudinal position of the fibre, so that in order to generate agrating the UV beam should be turned on and off as the fibre istranslated to align the interference pattern arising from successiveexposures through the phase mask.

[0036]FIGS. 2a to 2 c are schematic diagrams showing a gratingfabrication process by repeated exposures of the fibre to the UV beam.

[0037] In FIG. 2a, the UV beam from the acousto-optic modulator 50passes through the phase mask 30 to impinge on the fibre 10. During theexposure process, the fibre 10 is being longitudinally translated by thetranslation stage 20 in a direction from right to left on the drawing.FIG. 2a illustrates (very schematically) a refractive index changeinduced in the fibre by a first exposure through the phase mask.

[0038]FIGS. 2a to 2 c illustrate a feature of the normal operation of aphase mask of this type, namely that the pitch of the lines or fringesof the interference pattern projected onto the fibre (which gives riseto the lines of the grating) is half that of (i.e. twice as close asthat of) the lines physically present (e.g. etched) in the phase mask.In this example, the phase mask has a “physical” pitch of 1 μm, and thelines projected onto the fibre have a pitch of 0.5 μm.

[0039] The UV beam is modulated by the acousto-optic modulator in aperiodic fashion synchronised with the translation of the fibre. In thisway, successive exposures, such as the two subsequent exposures shown inFIGS. 2b and 2 c, generate periodic refractive index changes alignedwith and overlapping the first exposure of FIG. 2a. Thus, the refractiveindex change providing each individual grating “element” or fringe isactually generated or built up by the cumulative effects of multipleexposures through different parts of the phase mask as the fibre movesalong behind the phase mask. This means (a) that the optical powerneeded to generate the grating can be distributed between potentially alarge number of exposures, so each exposure can be of a relatively lowpower (which in turn means that the output power of the laser 40 can berelatively low); and (b) the grating can be apodised by varying therelative positions of successive exposures (this will be described belowwith reference to FIG. 3b).

[0040] Although each of the successive exposures of the fibre to UVlight through the phase mask 30 could be a very short pulse (to “freeze”the motion of the fibre as the exposure is made), this has not provednecessary and in fact the present embodiment uses an exposure duty cyclein a range from below 10% to about 50%, although a wider range of dutycycles is possible. An example of a simple regular exposure duty cycleis shown schematically in FIG. 3a, which in fact illustrates the stateof the modulation control signal switching between an “on” state (inwhich light is passed by the acousto-optic modulator) and an “off” state(in which light is substantially blocked by the acousto-opticmodulator). The period, τ, of the modulation corresponds to the timetaken for the fibre 10 to be translated by one (or an integral number)spatial period of the interference pattern generated by the phase mask30.

[0041] As the duty cycle for the UV exposure increases, the gratingcontrast decreases (because of motion of the fibre during the exposure)but the writing efficiency increases (because more optical energy isdelivered to the fibre per exposure). Thus, selection of the duty cycleto be used is a balance between these two requirements.

[0042] Assuming linear growth, the index modulation, n_(g)(z) in anideal grating can be described as a raised cosine profile:

[0043] n_(g)(z)∝1+sin(2πz/Λ)

[0044] where z is the position down the fibre and Λ the grating period.With the new technique we obtain:

[0045]n_(g)(z)∝(ΔΛ_(ON)/Λ)[1+{sin(πΔΛ_(ON)/Λ)/(πΔΛ_(ON)/Λ)}sin(2π(z+ΔΛ_(ON)/2)/Λ)]

[0046] where ΔΛ_(ON)/Λ is the fraction of the period that the beam is on(i.e. the duty cycle).

[0047] For small values of ΔΛ_(ON)/Λ a near 100% grating contrast isobtained however the efficiency of the grating writing is reduced to˜ΔΛ_(ON)/Λ because most of the UV beam is prevented from reaching thefibre.

[0048] The maximum grating strength is obtained for ΔΛ_(ON)/Λ=0.5however the ratio of dc to ac index change is worse. For ΔΛ_(ON)/Λ>0.5the grating begins to be reduced whilst the dc index change continues tobuild.

[0049] Experimentally, a good value for ΔΛ_(ON)/Λ has been found to be˜0.3-0.4.

[0050] Thus, with embodiments of this technique, exposure of the gratinglines or elements is repeated every grating period. Thus the footprintdefined by the UV beam, which might for example for a 500 μm diameterbeam, φ_(beam), consists of φ_(beam)/Λ(˜1000) lines, is significantlyoverlapped with the previously exposed lines. Significant averaging ofthe writing process given by (φ_(beam)/Λ)^(½) is therefore achieved,thus improving the effective accuracy and resolution of the system.

[0051] The computer in this embodiment actually generates the switchingpositions internally as “real” numbers (obviously subject to thelimitation of the number of bits used), but then converts them foroutput to the controller into the same unit system as that output by theZygo interferometer, namely multiples of a “Zygo unit” of 1.24 nm. Thisinternal conversion by the computer makes the comparison of the actualposition and the required switching position much easier and thereforequicker for the controller. A random digitisation routine is employed inthe computer 60 to avoid digitisation errors during the conversion fromreal numbers to Zygo units. This involves adding a random amount in therange of ±0.5 Zygo units to the real number position data before thatnumber is quantised into Zygo units. Thus an effective resolution can beobtained of:

[0052] 1.24 nm/(φ_(beam)/Λ)^(½)≈0.03 nm.

[0053] The technique offers the further advantage that the CW laser isextremely stable whereas pulsed lasers (as required in the techniqueproposed by Stubbe et al [7]) may suffer from pulse-to-pulse instabilitywhich, in the Stubbe et al technique, is not averaged over multipleexposures. In addition the high peak powers of a pulsed laser may causenon-linear grating writing effects, which are avoided or alleviated byusing longer and repeated exposures in the present technique.

[0054] A refinement of the above technique, for producing apodisedgratings, will now be described with reference to FIG. 3b.

[0055] Using the techniques described above, effectively controlling thegrating writing process on a line-by-line basis allows accurateapodisation to be achieved.

[0056] Apodisation is achieved by effectively dithering the gratingwriting interferometer position in the fibre to wash out or attenuatethe grating strength. However, if the overall duty cycle of the exposureis kept the same, and just the timing of each exposure dithered, theaverage index change along the grating is kept constant.

[0057] To completely wash out the grating subsequent on periods of theUV laser are shifted in phase (position) by ±π/2(±Λ/4). To achieve areduced attenuation the amplitude or amount of dither is reduced. FIG.3b illustrates an applied dither of about ±π/3 from the original(undithered) exposure times.

[0058] This technique of apodising is better with an exposure duty cycleof less than 50%, to allow a timing margin for 100% apodisation.

[0059] One example of the use of this technique is to generate a gratingwith a contrast increasing at one end of the grating according to araised cosine envelope, and decreasing at the other end of the gratingin accordance with a similar raised cosine envelope, and remainingsubstantially constant along the central section of the grating. Thisapodisation can be achieved particularly easily with the presenttechnique, as the central section requires no phase shift betweensuccessive exposures, and the two raised cosine envelopes require aphase shift that varies linearly with longitudinal position of thefibre.

[0060] The required phase shifts can be calculated straightforwardly bythe computer 60, under the control of a simple computer program relatingrequired phase shift to linear position of the fibre (effectivelycommunicated back to the computer 60 by the controller 70, whenever thecontroller 70 requests a next switching position data value).

[0061] Other apodisation schemes are also possible. Compared withprevious methods of dithering [6] this technique is not limited by thedynamics of a mechanical stage used for dithering, but instead simplyadjusts the switching time of a non-mechanical modulator element 50. Itcan also achieve substantially instantaneous phase shifts.

[0062] Furthermore, arbitrary phase profiles and in particular a linearchirp can be built up by the computer 60 inducing phase shifts along thegrating as it is fabricated. In a similar manner to the “Movingfibre/phase mask” technique [6] the maximum wavelength is inverselyproportional to the beam diameter. However, with the present techniquean improvement can be obtained (with respect to the technique of [6]) byincorporating a short, linearly chirped phase mask. Thus as the fibre isscanned the UV beam is also slowly scanned (by another PZT translationstage, not shown) across the phase mask. This scanning of the positionof the UV beam in itself induces a small chirp, in accordance with thetechniques described in reference [6], but more significantly thetranslated beam accesses writing lines of a different period allowinglarger chirps to be built up. This has been tested using a 19 mmdiameter, ˜20 nm chirped phase mask (sourced from Lasiris) with itscentral period around 1070 nm. This allows ˜30 nm chirped gratingscentred around a central wavelength of 1550 nm to be fabricated.

[0063]FIGS. 4a and 4 b are schematic graphs showing the characterisationof a 20 cm linearly chirped grating written at a fibre translation speedof 200 μm/s with the basic technique described earlier, i.e. with afixed mask. At this fibre translation speed, for a projected fringepitch of 0.5 μm the writing light beam is switched at a switching rateof 400 Hz. In other words, the fibre advances by one projected fringebetween exposures. (It is noted that the limitation on fibre translationspeed in these prototype experiments is the calculation speed of thecomputer 60 used in the experiments, and that given a faster computersuch as a Pentium or subsequent generation PC, much higher translationspeeds of, say, 10 mm per second or more would be possible).

[0064] In particular, therefore, FIG. 4a is a graph of reflectivityagainst wavelength, and FIG. 4b is a graph of time delay againstwavelength. The wavelength (horizontal) axes of the two graphs have thesame scale, which for clarity of the diagram is recited under FIG. 4bonly.

[0065] A ˜4nm bandwidth and dispersion of ˜500 ps/nm are observed.

[0066] Such results have not been reported by any other method. Gratingsup to 40 cm aend writing speeds up to 1 mm/s have been demonstrated.Lengths in excess of in and writing speeds up to 10 mm/s are feasible.

[0067] In the above description, the fibre has been translated withrespect to the phase mask, and in the later description the UV beam istranslated with respect to the phase mask. However, it will be clearthat the important thing is relative motion, and so the choice of whichcomponent (if any,) remains “fixed” and which is translated isrelatively arbitrary. Having said this, however, the arrangementdescribed above has been tested experimentally and has been found to beadvantageously convenient to implement. It will also be apparent that inother embodiments each “exposure” could in fact involve a group of twoor more exposures, with the position of the fibre with respect to thewriting light beam being constant or substantially constant forexposures within a group, but different from group to group.

Publication References

[0068] 1. D. Garthe et al, Proc. ECOC, vol. 4, (post-deadline papers),pp. 11-14 (1994).

[0069] 2. G. Meltz et al, Opt. Lett., 14(15), pp. 823-825, 1989.

[0070] 3. K. O. Hill et al, Appl. Phys. Lett., 62(10), pp. 1035-1037,1993.

[0071] 4. R. I. Laming et al, Proc.ECOC'95, Brussels, Vol 2, PaperWe.B.1.7, pp 585-8, Sep. 17-21 1995.

[0072] 5. R. Kashyap et al, Electronics Letters, Vol 32 (15), pp.1394-6, 1996.

[0073] 6. M. J. Cole et al, Electronics Letters, Vol 31 (17), pp 1488-9,1995.

[0074] 7. R. Stubbe et al, postdeadline paper 1, Proc. Photosensitivityand Quadratic Nonlinearity in Glass Waveguides, Portland, Oreg., Sep.9-11, 1995.

1. A method of fabricating an optical waveguide grating having aplurality of grating lines of refractive index variation, the methodcomprising the steps of: (i) repeatedly exposing a spatially periodicwriting light pattern onto a photosensitive optical waveguide (10); and(ii) moving (20) the writing light pattern and/or the waveguide (10)between successive exposures or groups of exposures of the writing lightpattern, characterised in that the successive exposures or groups ofexposures overlap so that each of at least a majority of the gratinglines is generated by at least two exposures to different respectiveregions of the writing light pattern.
 2. A method according to claim 1,in which step (i) comprises moving (20) the writing light pattern and/orthe waveguide (10) between exposures a by a distance, in a substantiallylongitudinal waveguide direction, substantially equal to an integralnumber of spatial periods of the writing light pattern.
 3. A methodaccording to claim 2, in which step (i) comprises moving (20) thewriting light pattern and/or the waveguide (10) between exposures a by adistance, in a substantially longitudinal waveguide direction,substantially equal to one spatial period of the writing light pattern.4. A method according to any one of claims 1 to 3, in which step (ii)comprises: detecting (55) the relative position of the writing lightpattern and the waveguide (10); comparing the detected relative positionto predetermined switching positions related to the spatial period ofthe writing light pattern; and controlling exposure of the writing lightpattern in response to that comparison.
 5. A method according to any oneof the preceding claims, in which: the writing light pattern isgenerated from one or more source light beams (40); and exposure of thewriting light pattern is controlled by directing the one or more sourcelight beams through one or more optical modulators (50).
 6. A methodaccording to claim 5, in which the writing light pattern is generated bydirecting the source light beam through a phase mask (30).
 7. A methodaccording to claim 5 or claim 6, in which the one or more source lightbeams are substantially continuously generated (CW) light beams (40). 8.A method according to any one of the preceding claims, in which step (i)comprises moving the writing light pattern and/or the waveguide (10) ata substantially uniform relative velocity.
 9. A method according toclaim 8, in which step (i) comprises substantially periodically exposingthe writing light beam onto the waveguide (10), the exposures having asubstantially constant temporal duty cycle.
 10. A method according toclaim 9, in which step (i) comprises varying the time at which eachexposure of the writing light beam is made to vary the spatial alignmentalong the waveguide (10) of successive exposures, thereby varying thecontrast of grating lines generated by those exposures.
 11. A methodaccording to any one of the preceding claims, comprising varying thespatial period of the writing light beam during fabrication of thegrating.
 12. A method according to claim 6 and claim 11, comprisingdirecting the source light beam onto different regions of a chirpedphase mask (30) in order to vary the spatial period of the writing lightbeam during fabrication of the grating.
 13. A method according to anyone of the preceding claims, in which the waveguide (10) is an opticalfibre.
 14. Apparatus for fabricating an optical fibre grating having aplurality of grating lines of refractive index variation, the apparatuscomprising: a writing light beam source (40) for repeatedly exposing aspatially periodic writing light pattern onto a photosensitive opticalwaveguide (10); and means for moving the writing light pattern and/orthe waveguide (10) between successive exposures or groups of exposuresof the writing light pattern, characterised in that the successiveexposures or groups of exposures overlap so that each of at least amajority of the grating lines is generated by at least two exposures todifferent respective regions of the writing light pattern.